1. Field of the Invention
The present invention relates to an image data processing method and an apparatus therefor for obtaining image data correctly reflecting the gradation of an original.
2. Description of the Prior Art
In an image data processing apparatus such as a process scanner or a fascimile, it is important to correctly read the gradation of an original image. FIG. 1 partially shows an example of a conventional reflection type optical image reading mechanism employed for such an image data processing apparatus. Referring to FIG. 1, an original 3 is placed in a downwardly directed state on the upper surface of a transparent original receiving glass plate 2 provided in an upper opening of frame box 1. Light 5 from a light source 4 is irradiated on the original 3 through the original receiving glass plate 2. Reflected light 10 from the original 3 enters a photoelectric converter, such as a CCD line sensor 8, thourgh an image-forming optical system 7 comprising mirrors, lenses and the like. Such a photoelectric converter may also be prepared by a photomultiplier or a photodiode. The CCD line sensor 8 generates an image signal V.sub.CCD which is responsive to the intensity of light 9 incident upon its light receiving surface. The image signal V.sub.CCD is subjected to desired image processing, to be employed for exposure/recording on a photosensitive material (not shown) or image data transfer.
In such an image reading mechanism, the incident light 9 is expected to reflect the gradation of the original 3 in fidelity if the light 9 received in the CCD line sensor 8 includes only the reflected light 10 from the original 3. In practice, however, the light 5 emitted from the light source 4 is reflected also by the lower surface of the original receiving glass plate 2. Therefore, light 6 relected by the glass plate 2 is also included in the incident light 9 upon the CCD line sensor 8. Further, flare light (not shown) scattered through the inner wall surface of the frame box 1, other portions of the apparatus and the like is also received in the light receiving surface of the CCD line sensor 8.
Therefore, the image signal V.sub.CCD has background components irrelevant to image data of the original 3 by background light such as the reflected light 6 from the original receiving glass plate 2 and the flare light (stray light). As a result, it is difficult to correctly detect the gradation of the original 3 in the apparatus employing such a mechanism.
Description is now made on the influence exerted by such background light in the case of a process scanner. In the process scanner, an operator selects highlight and shadow points from bright and dark parts of the original 3 respectively, as is well known in the art. Optical density levels of these points are measured and halftone area rates for tint generation are designated with respect to the highlight and shadow points. A gradation correction curve (gradation curve) defining a relation between the optical density and the halftone area rate is determined on the basis of these data. The inputted image signal is subjected to shading correction, to be further subjected to gradation correction along the gradation correction curve. As a result, an exposure/recording signal responsive to the halftone area rate is obtained.
When the background light is present, however, it is impossible to obtain the gradation expression intended by the operator in image recording. In the case of positive recording, for example, the recorded image is entirely shifted to a brighter side (lower optical density side) from the original image.
Particularly in a dark side portion of the gradation curve, the amount of the background light is substantially equal to or larger than the amount of the light from a dark portion of the original, whereby the effect of by the background light on gradation expressibility is relatively increased. Thus, gradation expressibility in the dark portion is deteriorated in a positive recorded image. A similar problem takes place in a bright portion in the case of a negative recorded image.
In order to overcome the problem, there has been proposed an attempt to prevent generation of such background light itself. In practice, however, such an attempt has little implemetability in technique and cost, and hence the same cannot suitably cope with the problem.